The invention relates to a slip ring bushing for transmitting electrical energy between two relatively rotatable surfaces. The invention also relates to a method of inserting electrically conductive balls between two radially spaced annular surfaces.
Heretofore, in machines such as electric rotating machines, which utilize a power-driven rotor shaft (such as electric motors, generators, etc.), it has been conventional to transmit electrical energy between relatively rotatable surfaces by a slip ring apparatus employing a ring and a sliding brush. However, substantial frictional wear results, shortening the life of the apparatus.
It has also been proposed in U.S. Pat. No. 2,409,600 to replace brushes with electrically conductive rolling elements, such as spheres or cylinders which roll within an annular space formed between radially spaced outer and inner electrically conductive surfaces. The rolling elements are rotatably mounted in a cage, preferably formed of an electrically insulative material, which holds the rolling elements in circumferentially spaced apart relationship. As the inner ring rotates about a horizontal axis, the rolling elements roll against the outer and inner surfaces and conduct electrical current therebetween. In the case where spherically shaped rolling elements are used, the outer and inner surfaces are correspondingly shaped, and the outer surface is formed of half-sections that are bolted together to facilitate assembly of the parts. In addition, U.S. Pat. No. 4,592,605 discloses the use of caged rolling elements to transfer electrical current.
A shortcoming of the above-described structures is that it is virtually impossible to make all of the rolling elements of exactly the same diameter or to make the diameter exactly equal to the radial spacing between the inner and outer segments. Thus, it is possible that none of the rolling elements will actually contact both of the outer and inner surfaces. If the cage were formed of an electrically conductive material, an electrical transfer could take place through the cage, i.e., at a first interface between a rolling element touching one of the inner and outer conductive surfaces, then at a second interface between that rolling element and the cage, then at a third interface between the cage and a rolling element touching the other of the inner and outer conductive surfaces, and lastly at a fourth interface between the latter rolling element and the conductive surface. That travel path is relatively extensive and involves considerable electrical resistance, especially at the four interfaces where there may not be firm surface-to-surface contact, because in the absence of firm contact the electrical resistance at the interfaces is significantly increased. As a result, the current-carrying capacity of such a structure may be severely limited.
Another prior art proposal involves the provision of rolling elements in the form of circumferentially spaced, elastically flexible loops which are intended to flex under the forces imposed thereon by the electrically conductive surfaces. The loops thus roll between those outer and inner conductive surfaces while in a compressed state, and wherein rolling sleeves formed of an electrically conductive material are maintained in contact with circumferentially adjacent loops (see U.S. Pat. No. 4,372,633). However, the elastic force of the loops against the outer and inner surfaces increases the rate of wear.